In earth observation satellite and aerial imaging, current sensor technology allows to capture high resolution images at high speed, creating vast amounts of data. To cover large areas without gaps, series of images are taken at regular time intervals (expressed as frame rates), while the airborne platform moves forward with an intentional nominal forward speed. However, platforms unavoidably also exhibit some unwanted movement known as platform attitude variation. This influences the pointing angles, and thus the imaged areas, in a generally unpredictable way. To avoid gaps in the imaging, frame rates are usually increased, which in turn increases the data volumes. The actually required data volumes may exceed the downlink and/or storage capacity of aircraft-mounted systems. This problem occurs in all imaging systems, including panchromatic imaging systems as well as multispectral imaging systems (whether mosaic-based, or line-based as described below).
The limits of the downlink and/or storage capacity of the systems become particularly pressing in the case of multispectral imaging, where the total amount of data is multiplied by the number of spectral bands, and a fortiori in the case of hyperspectral imaging. Hyperspectral imaging is a form of spectral imaging wherein information from across the electromagnetic spectrum is collected in many narrow spectral bands and processed. From the different spectral images that are collected, information of the objects that are imaged can be derived. For example, as certain objects leave unique spectral signatures in images which may even depend on the status of the object, information obtained by multi-spectral imaging can provide information regarding the presence and/or status of objects in a region that is imaged. After selection of a spectral range that will be imaged, as spectral images in this complete spectral range can be acquired, one does not need to have detailed prior knowledge of the objects, and post-processing may allow to obtain all available information.
In some applications, multi-spectral data can be obtained by collecting a full two dimensional image of a region in one spectral range of interest and by subsequently collecting other full two dimensional images of that region in other spectral ranges of interest whereby spectral filters are switched in between. This way of data collection nevertheless is not always possible, especially when the region of interest and the imaging system undergo a large relative movement with respect to each other.
International patent application publication WO 2011/073430 A1, in the name of the present applicant, discloses a sensing device for obtaining geometric referenced multi-spectral image data of a region of interest in relative movement with respect to the sensing device. The sensing device comprises a first two dimensional sensor element. The sensing device is adapted for obtaining subsequent multi-spectral images during said relative motion of the region of interest with respect to the sensing device thus providing spectrally distinct information for different parts of a region of interest using different parts of the first sensor. The sensing device also comprises a second two dimensional sensor element and is adapted for providing, using the second sensor element, an image of the region of interest for generating geometric referencing information to be coupled to the distinct spectral information.
While these arrangements are very compact, they require a very high image capturing frame rate to ensure sufficient spatial resolution over the entire desired spectral range. Accordingly, there is a need for efficient use of downlink and/or storage capacity of the described systems.